The phosphorylation of proteins and lipids is an important cellular regulation mechanism which plays a role in many different biological processes such as cell proliferation, differentiation, apoptosis, metabolism, inflammation, immune reactions and angiogenesis. More than 500 kinases are encoded in the human genome. In general, tyrosine protein kinases are stimulated by growth factors or other mitogenic signals and phosphorylate proteins which initiate rapid signal transmissions. Serine/threonine protein kinases mostly phosphorylate proteins which crosslink and amplify intracellular signals. Lipid kinases are likewise important switching sites in intracellular signal pathways, with these sites linking various biological processes.
A number of protein kinases have already proved to be suitable target molecules for therapeutic intervention in a variety of indications, e.g. cancer and inflammatory and autoimmune diseases. Since a high percentage of the genes involved in the development of cancer which have been identified thus far encode kinases, these enzymes are attractive target molecules for the therapy of cancer in particular.
Phosphatidylinositol 3-kinases (PI3 kinases) are a subfamily of the lipid kinases and catalyse the transfer of a phosphate radical to the 3′ position of the inositol ring of phosphoinositides. They play a crucial role in a large number of cellular processes such as cell growth and differentiation processes, the regulation of cytoskeletal changes and the regulation of intracellular transport processes. The PI3 kinases can be subdivided into different classes on the basis of their in-vitro specificity for particular phosphoinositide substrates.
Among the members of the class I PI3 kinases, the α, β and δ PI3 kinases (class IA) are principally activated by receptor tyrosine kinases (RTKs) or soluble tyrosine kinases. On the other hand, the γ PI3 kinase (class IB) is principally activated by Gβγ subunits which are released from heterotrimeric G proteins following activation of heptahelical receptors. As a result of these differences in the coupling to cell surface receptors, in combination with a more or less restricted expression, the 4 class I PI3 kinases inevitably have very different tasks and functions in the intact organism.
Many independent findings indicate that class IA PI3 kinases are involved in uncontrolled processes of cell growth and differentiation. Thus, the first PI3 kinase activity which was detected was associated with the transforming activity of viral oncogenes such as the middle T antigen of polyoma viruses, Src tyrosine kinases or activated growth factors (Workman, Biochem Soc Trans. 2004; 32(Pt 2):393-6). Akt/PKB, which is activated directly by the lipid products of the class I PI3 kinases and in this way transmits the signals into the cell, is found to be hyperactive in many tumours such as breast cancer and ovarian or pancreatic carcinoma. In addition, it has recently been found that the PIK3 CA gene, which encodes the p110 subunit of PI3Kα, exhibits a high frequency of mutation in many tumour types such as colon, mammary and lung carcinomas, with some of the mutations being representatively characterized as being activating mutations (Samuels et al., Science 2004; 304(5670):554).